There are a number of integrated optical signal processors and fiber optic devices currently being developed or proposed that require a semiconductor diode laser source operating in a single longitudinal and fundamental transverse mode. Some examples of applications include an integrated optical RF spectrum analyzer, integrated optical and bulk optical wideband correlators, and laser fiber gyroscopes. Moreover, modulated versions of the single mode diode laser are required for long haul, single mode, fiber optic data links operating at very high data rates (1 Gbps), both because the narrow spectrum permits optical pulse propagation with a minimum of temporal dispersion and the optical response of such a laser diode is essentially free of relaxation oscillations. In integrated optical or bulk optical single processor applications, multimode oscillation either longitudinal or transverse leads to loss of resolution and reduced dynamic range. In fiber optic applications, multi-transverse mode operation results in reduced efficiency and coupling to single mode optical fibers and increased pulse dispersion. Therefore, there has been required the development of a single longitudinal and single transverse mode laser diode that can be mounted in a rugged, environmentally stable package capable of long life at room temperature and that can operate at an exceptionally low threshold and high efficiency.
The technology of the growth of gallium arsenide double heterostructure laser diodes has progressed to the point where a reasonably uniform state-of-the-art now exists among a number of sources of laser diode material. High quality double heterostructural structure wafers fabricated by liquid phase epitaxy yield broad area current thresholds of about 1000 A/cm.sup.2. At least five manufacturers (ITT Electro-Optical Products Division, RCA Incorporated, Laser Diode Laboratories Incorporated and Spectronix Incorporated) offer CW laser diodes with oxide striped current confinement having nearly identical current and output specifications. These diodes operate at threshold currents of 100 to 200 mA and 10 mW output when operated 100 mA over threshold. In spite of this uniformity in operating parameters, none of these available diodes are acceptable for use in many critical integrated optical circuit or single mode fiber applications. The difficulty is that two critical areas, the achievement of reliable single longitudinal mode oscillation, i.e., single frequency operation, and adequate control of the diode transverse spatial mode pattern are lacking in these devices.
Several diode manufacturers have sought to get around these problems by reducing the area of current confinement through the introduction of buried mesa or etch well of a width which is typically 0.5 to 2 .mu.m. The disadvantages of this procedure are the increased cost and the reduction of diode output power to only 1 or 2 mW. An advantage does result for end fire coupling into a fiber in that a more symmetrical beam shape is produced but part of this advantage is negated by the low numerical aperture of the single mode fibers into which this type of diode is usually coupled. Conversely, integrated optical circuits naturally accommodate a nonsymmetric output beam. The optical power required by integrated optical circuits is generally several milliwatts, whereas that required for long fiber optic links having high data rates and low bit error rates can exceed 10 mW. Each requires a laser diode of 10 to 25 mW output power when coupling losses are considered.
Heretofore the solutions to the difficult technical problem of achieving a reliable single mode oscillation, i.e., single frequency operation, involve the use of distributed feedback structures in the laser cavity to act as frequency sensitive filters or the control of doping levels in the active region. However, because of the losses inherent with the reported techniques, they are not consistent with low threshold CW room temperature operation with high efficiency. The other major problem area has been the control of diode transverse spatial mode pattern. Many exotic resonator designs heretofore reported for transverse mode selection employ tilted or curved stripes or filamentary resonators. These techniques increase resonator loss and hence adversely affect lasing threshold and efficiency. Additionally, the required packaging and environmental stability has been lacking in the prior art. However, this in some measure can be overcome through the use of the concepts embodied in U.S. Patent Application Ser. No. 069,311 entitled "Heat Sink Laser Diode" by Allen et al, filed Aug. 24, 1979 and assigned to Applicants' assignee.
Typically, to create a laser diode one must have a PN junction in which electrons are injected across the junction followed by recombination of electrons and holes to generate photons. For lasing to occur, a spontaneous emitted photon must transverse the cavity formed by the semiconductor material and the natural mirrors at the opposite ends thereof with sufficient gain to avoid attenuation. In a heterostructure laser diode, the carriers are confined in an active region by a potential barrier to reduce the temperature dependence of the threshold as well as its value. The barrier is created by putting in material with a higher energy gap near the junction. The most widely used of such materials are gallium arsenide and aluminum arsenide.